Question

On a topographical map with a scale of $1 : 25,000$ and a contour interval of $20\text{ m}$, a river flows down a valley. Contour line $V_1$, whose V-shape apex points upstream toward higher elevation, crosses the river channel at an elevation of $360\text{ m}$. Further downstream, the river crosses contour line $V_2$, which is separated from $V_1$ by $4$ contour intervals. If the measured map distance along the stream between these two crossing points is $8\text{ cm}$, what is the average gradient of the river channel between $V_1$ and $V_2$?